United States
Criminal investigations in the United States
Who investigates crime in a country with no national police: state and local agencies for most crime, federal investigators for federal offences, and the prosecutor’s role.
Facts on this page were checked against their sources on .
Most criminal investigation in the United States is done by state and local agencies — the 17,541 general-purpose state and local law-enforcement agencies counted by the Bureau of Justice Statistics — because most crime is a matter of state law. There is no national police force with general investigative authority over all crime.
Federal investigative authority is tied to federal offences. The statutory basis for federal investigators (28 U.S.C. § 533) authorises the appointment of officials "to detect and prosecute crimes against the United States", and preserves the investigative authority assigned by law to other agencies. Federal investigators therefore work within enumerated federal competence rather than across all crime.
The U.S. Marshals Service, the enforcement arm of the federal courts, is the federal government’s primary agency for fugitive investigations and holds the broadest arrest authority among federal law-enforcement agencies; it assists state and local agencies in locating and apprehending their most violent fugitives. Assistance across levels is routine; a command relationship across them is not.
The same idea, explained generally
- What is a criminal investigation?
A criminal investigation is a legally authorised process for establishing whether an offence occurred and gathering evidence that can be tested in court. This guide explains its structure, safeguards, and known failure modes.
- What is due process?
Due process is the requirement that the state follow fair, established procedures before it takes action against a person. This guide explains what it typically includes and how it differs between systems.
Sources
Sources
28 U.S. Code § 533 (Investigative and other officials; appointment)
Legal Information Institute, Cornell Law School (reproducing the United States Code) · Legislation · link verified 2026-07-24
Verbatim: the Attorney General may appoint officials '(1) to detect and prosecute crimes against the United States'. The statutory basis for federal investigative authority (the FBI is established under this and related provisions). Supports that federal investigative authority is tied to crimes AGAINST THE UNITED STATES — i.e. federal offences — and preserves the investigative authority assigned by law to other agencies. Does NOT support any claim of general federal jurisdiction over all crime, nor any operational detail.
What We Do — Duties and Operations, U.S. Marshals Service
United States Marshals Service (usmarshals.gov) · Government publication · link verified 2026-07-24
The agency's own description. Supports that the U.S. Marshals Service is the enforcement arm of the federal courts, and that its duties include protecting the federal judiciary, apprehending federal fugitives (with the broadest arrest authority among federal law-enforcement agencies), housing and transporting federal prisoners, managing seized assets and operating the Witness Security Program; and that it ASSISTS state and local agencies with fugitives rather than commanding them. Supports the federal judicial-enforcement role only; not operational detail.
Census of State and Local Law Enforcement Agencies, 2018 — Statistical Tables
Bureau of Justice Statistics, U.S. Department of Justice · Government publication · 2018 · link verified 2026-07-24
Reference date June 2018. Supports that 17,541 general-purpose state and local law-enforcement agencies performed law-enforcement functions in the United States, of which about 67% were local (municipal) police departments, 17% sheriffs' offices, and 15% primary state, tribal, special-jurisdiction agencies, constables and marshals, together employing about 1,214,000 full-time personnel. Supports the FRAGMENTATION of US policing across thousands of separately governed agencies. Does NOT support any per-state figure or any claim about a specific agency.
Tenth Amendment to the Constitution of the United States
Legal Information Institute, Cornell Law School (reproducing the enacted constitutional text) · Legislation · 1791-12-15 · link verified 2026-07-24
Verbatim text: 'The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.' Supports the reserved-powers structure of US federalism — that the states, not the federal government, hold residual authority. Cornell LII reproduces the enacted text; the constitutional text itself is authoritative. Does NOT support any claim about how a particular power is allocated in practice.